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The ability of d(42 Mev)-Be neutrons and 250 kV X rays to produce sister-chromatid exchanges (SCE) has been reexamined using unstimulated (G0) human-blood lymphocytes. Contrary to a previous report, the neutrons produced a significant and dose-dependent increase in SCE. X rays, as previously, produced no measurable increase at any absorbed dose. The relative biological effectiveness (RBE) for this end-point is therefore undefined and effectively infinite. In contrast to the findings of many workers, the between-cell distributions of SCE were, in most cases, underdispersed. This could imply that the SCE burden of a cell was not selectively neutral in these experiments.

Graded X-ray doses up to 6.0 Gy were delivered to the rear 1/3 of body length (? 1/3 body mass) of Syrian hamsters and 24 h later, peripheral blood lymphocytes were sampled, set up in culture together with bromodeoxyuridine and first poststimulation mitotic cells were scored for chromosome-type aberrations. Very few aberrations were found at any dose and the response curve for centric exchanges (“dicentric + centric rings”) had no significant curvature or slope.Single irradiations of 3.0 Gy delivered to the whole body and sampled by identical protocol produced yields identical to, or slightly exceeding, those found for blood irradiated with the same dose in vitro.The same dose delivered to the front 2/3 or front 1/3 (head) or localised to the spleen area, and aberrations scored from blood or spleen-pulp lymphocytes, gave variable yields, but always higher than those obtained from the rear 1/3 irradiations.The results are consistent with dose-dependent interphase death of irradiated cells, coupled with a non-uniform distribution of lymphocytes within body tissues. It is estimated that some 80–90% of the lymphocytes are resident within the anterior 2/3.Detailed studies of the between-cell aberration distributions give evidence that positive selection against cells with high aberration frequencies has also occurred in these experiments.

No one can read the papers of the early radiation biologists without being impressed, almost awed, by the depth and thoroughness with which they tackled their topics. Every ounce of information is extracted from the results, every possible avenue of interpretation and implication is explored.Douglas Lea's Actions of Radiations on Living Cells(first edition, 1946) is a classic example. It is a work that has had a profound influence on radiobiology and is still widely quoted today.Many of Lea's fundamental concepts were based on the qualitative and quantitative production of visible chromosome structural changes, particularly in the plant Tradescantia.In accord with Sherlock Holmes' aphorism, before theorizing, he summarizes and evaluates in Chapter VI the experimental facts. In the intervening years an enormous amount of radiation cytogenetics has been done and it is fair to ask the question “How stands the foundation?”. I do not intend to review the whole field in this paper but, using the same section headings Lea used, to comment on and update, in the light of more recent work, some of the statements and ideas he developed. The format of the comments presupposes that the reader has a copy of Lea (1946) to hand.†Before looking at the individual sections, it will be helpful to describe some of the conceptual changes that have occurred since Lea wrote his book and that influence current thinking.